Kuwait summoned Tehran's envoy on Friday to demand the release of four Kuwaitis arrested in southwestern Iran, the foreign ministry said.

The ministry said it was "following up closely with Iranian authorities" to secure the release of the four nationals detained in the Ahvaz area, according to a statement carried by KUNA state news agency.

It said that the Iranian ambassador was handed a memorandum "demanding their release and allowing visitation" by Kuwaiti diplomats.

Kuwait's embassy in Tehran has been in contact with Iranian authorities to find out the reason behind the arrest of its nationals, the statement said.

It was not clear when the four were arrested.

Kuwait recalled its ambassador from Tehran in January after relations were broken off between Saudi Arabia and Iran following attacks on the kingdom's missions.

A Kuwaiti and an Iranian were sentenced to death in January by a court in Kuwait for spying for Iran and Lebanon's Shiite group Hezbollah.